1. Field of the Invention
The invention relates to the field of lighting devices, and more particular to that of motor vehicle headlights. Its object is a device for cooling one or a plurality of optical modules which equip a headlight of this type, for emission of various light beams. More specifically, this cooling device is of the type which induces heat exchange by convection between this flow of air and the heat-exchange surface of a cooling unit, such as a heat dissipater with fins or the like, which equips the optical module(s).
2. Description of the Related Art
In general, motor vehicle headlights consist of a housing which is closed by a transparent wall, through which one or a plurality of light beams are emitted. This housing accommodates at least one optical module, comprising mainly a source of light and an optical system which can modify at least one parameter of the light which is generated by the source of light, for emission of the light beam by the optical module. The optical system comprises optical components such as a reflector, a lens, a diffusing element or a collimator, or any other unit which can modify at least one of the parameters of the light generated by the source of light, such as its mean reflection and/or its direction.
The development of the technology is tending to favor the use of sources of light constituted by at least one LED (Light Emitting Diode), because of the low energy consumption of these lights and the quality of the lighting obtained. LEDs do not radiate in an omni-directional manner, but in a more directive manner than other sources of light. The small size of LEDs and their directive radiation of light make it possible to reduce the dimensions and simplify the structure of the optical module, with the advantage of facilitating integration of the latter in the interior of the housing. However, during functioning, the LEDs produce heat which is detrimental to their operation, since the more the temperature of an LED increases, the more its flow of light decreases. It is therefore necessary to make provisions for discharging the heat generated by the LED(s) which constitute the source of light of the optical module, in order to avoid an increase in temperature of the LEDs above a tolerable operating threshold.
For this purpose, the optical module is commonly equipped with a cooling unit in the form of a heat dissipater with fins, such as a radiator with fins or a similar heat-exchange unit. The cooling unit constitutes a support for the LED(s) installed on an electronic control board which is dedicated to each of the LEDs, or is common to all of them, or even to the optical system(s) at least partly, or commonly often in its/their entirety. The cooling unit makes it possible to discharge the heat which is generated by the source of light towards the interior volume of the housing and/or towards the exterior of the housing, on the basis of heat exchange which uses the surface of the fins which the cooling unit comprises. Optimization of the heat exchange between the dissipater with fins heated by the LEDs and the air can be obtained by increasing the surface area, by means of an increase in the size and/or number of fins of the cooling unit. However, this solution has the disadvantage of resulting in a consequent increase in the weight and overall dimensions of the optical module, which should be avoided in order to facilitate installation of the module inside the housing. Installation of this type may be made problematic because of shortage of space available for receipt of the optical module(s), and/or because of the constraints associated with the overall arrangement of the headlight in relation to its close environment when it is fitted on the vehicle. It is consequently advantageous to organize the cooling of the optical module(s) so as to avoid impeding its/their ease of installation inside the housing.
Account must also be taken of the fact that the volume of the means used for cooling of the LEDs which the optical modules comprise depends on the quantity of heat which they generate, according to the operating power, which itself is dependent on the intensity of light necessary for emission by the headlight of the corresponding light beam.
More particularly, an optical module or group of optical modules is organized in order to constitute a lighting device and/or signaling device which requires strong intensity of light, such as for a dipped beam, a full beam, a fog beam or a daytime signaling light. The number of LEDs and/or the power which is necessary in order for them to function is high, and the cooling means which are used for optical modules of this type are designed to make it possible to discharge a substantial amount of heat generated by the LEDs. If only the air which is naturally present in the interior of the housing is used to obtain adequate cooling of the LEDs, the lighting and/or signaling device needs dissipation means with a substantial mass. In order to overcome this difficulty, it is known to use a fan or a similar unit which induces forced passage of the flow of air along the fins of the cooling unit which equips the optical modules. The use of such a flow of air with forced passage makes it possible to limit the heat-exchange surface, and therefore the size of the cooling unit, and consequently makes it possible to limit the overall size of the optical module or group of optical modules. By way of example, reference can be made to document WO2005116520 which describes arrangements of this type. Also by way of example, it is known to use a heat pipe which can convey heat by conduction from the material which constitutes it, and/or which is placed in a hermetically sealed pipe which can convey a heat-exchanging fluid which it contains. The heat-exchanging fluid is, for example, water or any other fluid which can be used for heat exchange. The pipe is closed, for example, by closure of its ends, or by closure of the pipe in a loop on itself. One end of the heat pipe is in contact with the cooling unit which equips the optical module(s) in order to collect by conduction the heat which is produced by the source(s) of light, whereas its other end is in contact with a cooling unit which is placed on the exterior of the housing, in order to discharge the heat which is conveyed by the heat pipe. Reference can be made, for example, to documents EP1881262 or US2008/0025038 which describe arrangements of this type.
There is, therefore, a need to provide an improved device for cooling an optical module for a motor vehicle.